Question about the decay chain and how to choose the helicity of the decayed particle

Hi,

> My question is that how exactly does the MadGraph make it?

This is an intermediate particle, so we include the full propagator in the computation of the matrix-element

> Can we choose the helicity of the decayed particle?

No intermediate particle are not related to a given helicity.
In principle, you can change the propagator of the particle but this is not very practical in general.

> For example, p p > w+ w-, w+ > l+ vl, can we choose the helicity of w+ so that only the longitudinal polarisation of w+ is generated?

No.

Cheers,

Olivier

On 25 Apr 2015, at 11:01, Da Liu <email address hidden> wrote:

> New question #265859 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/265859
>
> Hi,
>
> The decay chain syntax restricts the particle to be on-shell and takes into account the spin-correlation with the decay products.
>
> My question is that how exactly does the MadGraph make it? Can we choose the helicity of the decayed particle?
>
> For example, p p > w+ w-, w+ > l+ vl, can we choose the helicity of w+ so that only the longitudinal polarisation of w+ is generated?
>
> Thank you very much.
>
> Best wishes,
> Da Liu
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Any chance that now this can be done using the "bias" function?
http://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/LOEventGenerationBias

or even the
# handling of the helicities:
# 0: sum over all helicities
# 1: importance sampling over helicities
#*********************************************************************
   0 = nhel ! using helicities importance sampling or not.
#*********************************************************************

I am also willing to edit the fortran code if necessary ... where one should fix the helicity of the W in the fortran?

Hi,

The "handling of helicities" only handle final state polarization (and only choose between an exact sum and a Monte-Carlo over the polarization.

However if you look at this:
https://answers.launchpad.net/mg5amcnlo/+faq/2243
You will have a sneak peak of a feature that should arrive in a future version of the code and which should be able to do just that.

Cheers,

Olivier

Hi Olivier! thanks for picking this up.
This new feature seems very to the point.

In the meantime I was wondering if MadSpin can help by

1) generating a p p > w+ w-
2) read the LHE output and strip all the events which contain the unwanted polarizations (at the expense of statistics, of course), saving the new file in place of the old one
3) run decay_events on the stripped LHE file

I see that decay_events recomputes matrix elements for the process with and without decays, maybe here I should use the modification of matrix*.f ?

This will not preserve the polarisation

Cheers

Olivier

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________________________________
From: <email address hidden> on behalf of Roberto Franceschini <email address hidden>
Sent: Sunday, March 31, 2019 12:08 AM
To: Olivier Mattelaer
Subject: Re: [Question #265859]: Question about the decay chain and how to choose the helicity of the decayed particle

Question #265859 on MadGraph5_aMC@NLO changed:
https://answers.launchpad.net/mg5amcnlo/+question/265859

Roberto Franceschini requested more information:
Hi Olivier! thanks for picking this up.
This new feature seems very to the point.

In the meantime I was wondering if MadSpin can help by

1) generating a p p > w+ w-
2) read the LHE output and strip all the events which contain the unwanted polarizations (at the expense of statistics, of course), saving the new file in place of the old one
3) run decay_events on the stripped LHE file

I see that decay_events recomputes matrix elements for the process with
and without decays, maybe here I should use the modification of
matrix*.f ?

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You received this question notification because you are an answer
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